The present invention relates to the field of a continuous roll-to-roll printing method for manufacturing electronic devices. More particularly, the present invention relates to an ultra-precision register control method in a continuous roll-to-roll printing process for manufacturing electronic devices, by which additional register errors attributable to variations in the speed of upstream printing cylinders are compensated for and eliminated for enhanced accuracy by using a feedforward control logic.
Recently, attention has been focused on mass production of low-cost electronic devices through a continuous roll-to-roll printing process. The production of electronic devices through a conventional batch method did not exhibit high productivity due to an intermittent way of production and the complexity of a production process attributable to etching or the like.
By contrast, roll-to-roll production using a continuous process enables materials to be continuously produced, and directly prints ink that may include even metal nanoparticles, such as silver or nickel on a material, thus rapidly increasing production speed. Yet, there remains a problem to be solved before applying the same conventional roll-to-roll printing process used for printing a general media to a roll-to-roll printing for electronic devices, that is, the problem of printing precision. The precision of a conventional printing process is about one hundred microns, which is the limit of error that can be detected by human eyes. An electronic device, however, requires a printing precision of, typically, one to fifty microns or less depending on a specific field of application.
A typical printing process using a continuous process uses either a sectional type register controller or a compensator roll type register controller for correcting register errors. In a recent continuous printing process, a sectional type register controller is being more used.
FIGS. 1 and 2 schematically show the two types of controllers typically used in the prior art. FIG. 1 shows the construction of a the compensator roll type register controller 10, in which the compensator roll type register controller transfers a driving force using a single main motor 12 and a shaft 14, thus rotating respective printing cylinders 20. At each roller, a gearbox 16 is installed and all printing cylinders 20 are rotating with the same speed. Register errors, the errors in printing positions, are measured by sensors 27 shown in FIG. 2 at suitable positions 25 in respective spans, the intervals between adjacent printing cylinders, and controlled and compensated for by changing span lengths, or equivalently, by changing phase differences between printing cylinders, through the motion of the compensator rolls 30 installed between respective printing cylinders 20. This scheme, however, has a relatively low efficiency in the aspect of cost and spatial utility because it requires additional equipments, such as compensator rolls, main motors, gearboxes, and linear motion guides, to be installed.
Such disadvantages are overcome in a printing scheme using a sectional type register controller 10 shown in FIG. 2. In this scheme, the respective printing cylinders 20 are driven by individual drivers 24 and motors 25 to allow speed control of individual priming cylinders 20, so a main shaft and compensator rolls are not needed. The way of controlling register errors is also different from a conventional compensator roll type printer. In a printer system employing a sectional type controller, register errors are measured at predetermined positions 25 by register sensors 27 positioned behind printing cylinders 20, and relayed to the sectional register controller 10, which then generates register control signals 30 to compensate for the register errors. The register control signals 30 are transferred then to a main controller 28 that controls the velocity and phase of each printing cylinder 20, which then distributes individual signals 32 to the individual drivers 24 to change the speeds of individual printing cylinders 20 through the motors 25. In this scheme, the changes in the speeds or phases of printing cylinders are in proportion to the magnitudes of register errors.
There is another important, probably the most important, difference between the two printing schemes. In the conventional compensator roll type printer, the motion of compensator rolls, designed to compensate for a register error of a particular span, influences not only the length of that particular span, but the length of subsequent spans. In the sectional type printer, however, the speed variation inputted into a particular printing cylinder for the purpose of compensating for a register error associated with that particular printing cylinder, influences not only the phase of that particular printing cylinder, but also the phases of subsequent printing cylinders. Therefore, in this type of a printing system, even when a register error in the current span is compensated for by changing the speed of the printing cylinder associated therewith, another register error occurs in subsequent spans due to the very action of compensation performed for correcting the error in the current span, that is, changing the speed of the printing cylinder associated with the current span.
This kind of phenomenon for the sectional type printer is illustrated in FIG. 3 showing register errors Y2 and Y3 and variations in tension T2 and T3 generating, respectively, in the first span between first and second printing cylinders 1 and 2, and in the second span between second and third printing cylinders 2 and 3 when the speed of the second printing cylinder 2 is changed using a square-type pulse input, V2. It can be seen that a register error occurs in the first span as well as in the second span, and the two register errors have the same magnitude, but opposite directions.
In a typical printing system, register errors caused in respective spans are controlled by using only a feedback control method using, for example, a proportional-integral-derivative (PID) control algorithm in each printing cylinder. However, in a roll-to-roll printing process of electronic devices that requires ultra-precision register control, the use of a conventional feedback control method alone is not enough for realizing such a desired level of precision due to the register errors that will occur in subsequent spans, being caused by the compensations performed in previous spans to upstream printing cylinders.
Therefore, there is a need in the art of a roll-to-roll printing process of electronic devices to develop a register compensation control method that is capable of compensating for, in advance, the register errors occurring in subsequent spans due to the speed inputs into upstream printing cylinders in previous spans, which are inputted to compensate for register errors occurring in the previous spans.